Method of modifying a boundary region of a substrate

ABSTRACT

A method of modifying a boundary region ( 9 ) of a substrate ( 3 ) bounded by a surface ( 10 ), wherein an evacuated process chamber ( 2 ) is provided having a plasma source ( 4 ) for generating a directed plasma jet ( 5 ), and wherein furthermore a reactive component is supplied into the process chamber ( 2 ) with a flow of a predefined size, and wherein the substrate ( 3 ) is heated to a predefined reaction temperature, characterized in that the reactive component is diffusion-activated by the directed plasma jet ( 5 ) such that the reactive component diffuses into the boundary region ( 11 ) of the substrate ( 3 ) at a predefinable diffusion rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C.§119 of EuropeanPatent Application No. 12169320.4 filed on May 24, 2012, the disclosureof which is expressly incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not a applicable.

REFERENCE TO A COMPACT DISK APPENDIX

Not applicable,

BACKGROUND OF THE INVENTION

Surface finishing is understood as the most varied technical methods toenhance or modify the surface properties of substrates. Substantially,in this respect, the functional and decorative properties of the surfaceof a substrate are modified, with functional properties meaning theenhancement of surfaces, for example with respect to corrosionprotection or wear protection, or decorative properties meaning, forexample, the gloss level or the color. Known examples forsurface-finished products are plastic parts or metal parts, for exampleDVDs or tools whose scratch resistance is improved or the manufacture ofdirt-repellent surfaces on glass and ceramics (lotus effect),

Two process classes used for surface finishing are coating processes andthe modification of a layer region in the substrate. The main methods ofthe first class, that is the coating processes, are e.g. physical vapordeposition, chemical vapor deposition and thermal spraying, inparticular plasma spraying, but also classical methods such aslacquering. The term physical vapor deposition (PVD) in this respectdesignates a group of vacuum-based coating processes or thin filmtechnologies. In this respect, the coating material is transformed intothe gas phase using physical processes and the gaseous material iscondensed on the substrate, with the target film being formed.

In chemical vapor deposition (CVD), in contrast, the gas phase of asolid component is frequently deposited on the heated surface of asubstrate due to a chemical reaction. The method is usuallycharacterized in this respect by a chemical reaction in the processchamber or at the surface of the workpiece to be coated. The frequentlyhigh temperature load on the substrate represents a substantiallimitation. The thermal load can cause, among other things, distortionat workpieces or it can lie above the softening temperature of thematerial to be coated so that the method cannot be used or can only beused with restrictions.

In a further category of coating processes, thermal spraying, additionalmaterials, the so-called spray additives, are melted off, partiallymelted or fully melted inside or outside a spray torch, are acceleratedin the form of spray particles in a gas flow and are bombarded onto thesurface of the component to be coated. A layer formation takes placesince the spray particles flatten more or less in dependence on theprocess and on the material on an impact onto the component surface,adhere primarily due to mechanical fusion and so build up the spraylayer.

The units used for the different methods, for example vacuum depositionplant, sputtering plant, plant for CVD and thermal spraying plant, forexample therma plasma spraying units, are today used in a number ofareas of industrial production.

Typical substances include, for example, workpieces having curved ornon-curved surfaces, for example tools or cylinder running surfaces ofinternal combustion engines, a number of components and semi-finishedproducts on which a corrosion protection is, for example, applied bymeans of a thermal spraying method, but also planar substrates such aswafers and films onto which a coating is applied, that is inter aliaconductive or insulating layers for semiconductors such as solar cells.The applied layers can be used to make the surface resistant towardheat, mechanical, chemical or corrosive influences, to improve frictionor adhesion on the surface, to make the surface electrically and/orthermally insulating or conductive, from case to case, to make thesurface compatible for foodstuffs or for blood or tissue or to formseals and diffusion barriers, to name just a few typical applications.

The most varied plant and processes are already known from the prior artfor the reactive pretreatment of the outer surface, that is for example,for the activation of the surface prior to a coating process, and fordepositing thin films by means of a plasma. A thermal spraying processusing plasma is described in EP 2 107 131 A1. The process describedthere deals with a method for the coating and/or for the surfacetreatment of substrates by a plasma jet which is produced in a processchamber by means of a plasma source. In this respect, plasma gas isconducted through the plasma source, the plasma gas is heated by meansof electrical gas discharge and/or electromagnetic induction and/ormicrowaves and the plasma jet is directed onto a substrate introducedinto the process chamber.

The process is characterized in that a reactive component is injectedinto the plasma jet in liquid or gaseous form to coat a surface of thesubstrate or to pretreat the outer surface prior to the coating, that isto activate it, to achieve a better adhesion of the coating. Thereactive component is in this respect supplied to the plasma source ingaseous or liquid form. Possible activation processes to improve theadhesion of the coating on the surface of the substrate include, forexample, the heating, cleaning, etching, partial oxidation or partialnitration of the outer surface, e.g. by means of a plasma jet.

Substantial disadvantages of the process described in EP 2 107 131 A1are that the pretreatment, that is the activation, of the surface, andthe coating of the surface are very time-consuming.

If, in contrast, the substrate as such is to be modified beneath or deepin the surface, i.e. in a boundary region beneath the surfaces,diffusion processes well-known from the prior art are known such asnitriding, carburizing, nitrocarburizing, oxide nitriding, carbonizing,oxidizing or borizing. Diffusion methods are processes in which a layerregion in the substrate is modified, usually at elevated temperatures,that is e.g. by diffusing of, frequently, nitrogen or carbon, or otherreactive substances into the substrate surface, a modified layer isformed beneath the surface of the substrate. A disadvantage in thisrespect is that the required process times for the treatment of thesubstrate are very long, up to 100 hours in part, which is uneconomicand only of limited suitability for many industrial applications.

It can thus be stated in summary that the two discussed classes ofmethods admittedly provide possibilities for coating at the surface of asubstrate or for modification by diffusion in a layer region beneath thesurface of the substrate, but have the mentioned disadvantages for anefficient production on an industrial scale.

SUMMARY

It is therefore the object of the invention to provide a new diffusionprocess in which the disadvantages of the know processes and methods areavoided and to propose a diffusion method which is less time-consumingand thus more economic than, for example, classical nitriding,carburizing or nitrocarburizing.

This object is satisfied by the method defined in claim 1.

The dependent claims relate to particularly advantageous embodiments ofthe invention.

The invention thus relates to a method of modifying a boundary region ofa substrate bounded by a surface, wherein an evacuated process chamberis provided having a plasma source for producing a directed plasma jet,and wherein furthermore a reactive component is supplied into theprocess chamber with a flow of predefined size and the substrate isheated to a predefined reaction temperature. The reactive component isdiffusion-activated in accordance with the invention by the directedplasma jet so that the reactive component diffuses into the boundaryregion of the substrate at a predefined diffusion rate.

It is thus important for the invention that the reactive component isdiffusion-activated by the directed plasma jet such that the reactivecomponent is present in a very high concentration, in particular in theregion of the plasma jet, and diffuses into the boundary region of thesubstrate at a predefined diffusion rate. Diffusion activation is to beunderstood as a direct breaking or splitting open of the reactivecomponents, for example into individual atoms or ions. It isparticularly advantageous in this respect that the diffusion activationtakes place, in comparison with known processes, in the region of theplasma jet and at a high temperature so that a high concentration ofreactive components arises, in particular in the region of the plasmajet. Due to the high local concentration of the reactive component inthe region of the plasma jet, but also in the marginal region of theplasma jet or in the process chamber outside the plasma jet, an improveddiffusion behavior occurs, i.e. the reactive component diffuses fasterinto the boundary region of the substrate. The process times are thusreduced by a multiple and the invention makes it possible to employ thepresent method in industrial processes and to utilize it economically.

In contrast to the known method described in EP 1 160 224 B2, in whichan arc is ignited at a lower voltage, usually DC current, in a plasmareactor, and the reactive component is distributed uniformly in theprocess space, the invention utilizes a modified process of low pressureplasma spraying (LPPS) using a directed plasma jet to bring about adirected diffusion process at specific boundary regions of thesubstrate. The diffusion dynamics are thus decisively improved by thepresent invention because a direct increase in the concentrations atreactive components, in a predefined diffusion rate and predefinedprocess parameters, is achieved or improved by the use of the modifiedLPPS principle.

In practice, the reactive component is liquid and/or gaseous and/orpowdery and/or a suspension, with the reactive component for diffusionactivation preferably being injected into the plasma jet of the plasmasource and/or into the process chamber. If the reactive component ispresent as a suspension or in liquid or gaseous form, three preferredembodiment variants are advantageously possible, either the injection ofa reactive component into the plasma jet in the plasma source or intothe free plasma jet or into the process chamber.

Depending on the embodiment variant, an injector is provided in theplasma source to inject a reactive component into the plasma jet. Theinjector can be arranged in the region of a nozzle which is provided forforming the plasma jet in the plasma source. The reactive component can,however, also be injected into the free plasma jet by means of aninjector, for example by means of an injector which is arranged at aspecific distance from the nozzle outlet opening of the plasma source orby means of an injector which injects the reactive component into theprocess chamber or is otherwise introduced into the process chamberand/or into the plasma jet. As long as the plasma jet is not fanned outa lot, the injector is advantageously arranged substantially centrallyin the plasma jet. If the plasma jet is fanned out more, for example ata distance of typically more than 0.1 m from the plasma source,ring-shaped injectors can, for example, also be provided.

If the reactive component is present as a powdery reagent, in a furtheradvantageous embodiment, the plasma source is provided with one or moreinfeeds to supply these powdery solid particles, but also a suspension,and thus to modify the boundary regions of the substrate by means of themodified LPPS process in accordance with the invention.

The infeed can, for example, be conducted up to and into the region of anozzle which is provided for forming the plasma jet in the plasma sourceto introduce reactive additives in the form of liquid or gaseousreactive elements or as powdery solid particles and/or as suspensionsinto the plasma jet at this point. The powdery solid particles are inthis respect advantageously supplied by means of a conveying gas.

The reactive component, which can include a hydrocarbon compound and/oroxygen and/or nitrogen and/or another reactive substance, diffusesthrough the surface into the boundary region of the substrate and in sodoing a compound, in particular a nitride or a nitro compound, acarbide, a nitrocarbide, a silicide, a carbonitride, an oxide, a borideor an aluminide is created.

Possible modifications of the boundary region of the substrate thusinclude nitriding, carburizing, aluminizing, nitrocarburizing, oxidenitriding, carbonitirding, oxidizing or borizing the substrate. Thedecisive difference from coating the surface of the substrate is in thisrespect that actually not coating is produced which is sufficientlyknown from the prior art, but rather a reactive component isdiffusion-activated and a boundary region in the substrate is modifiedby diffusion processes by means of a modified LPPS process. Someembodiments of modification processes which were manufactured using theabove-described method will be explained in more detail in thefollowing.

As a specific advantageous measure, the process chamber includes a heatsource to be able to carry out the modification at a reactiontemperature within a predefined temperature range, for example to beable to directly control the diffusion. As a further measure, thesubstrate can be preheated to a reaction temperature by means of theadditional heat source and/or the reaction temperature can additionallyor alternatively be controlled or regulated by means of the plasma jetduring the modification.

Since the substrate can be preheated before the modification_(;) forexample, in dependence on the embodiment variant, a heat source can beadvantageously provided for the process chamber or in the processchamber for the substrate. The process parameters such as the diffusionrate or the diffusion speed, etc. can thus be ideally set prior to themodification and to improve the modification of the boundary layer, thatis to achieve an economically efficient process and to achieve shortmodification and/or process times. The preheating of the substrate canin this respect take place using the same process parameters as themodification of the substrate, with it normally being sufficient to leadthe plasma jet, possibly not containing any reactive component forpreheating, over the substrate e.g. with a few pivot movements. In theideal case, the heat source is not required or is only required for ashort period and the plasma jet contains the reactive component so thatjust a few pivot movements are sufficient to heat the substrate surfaceto a required reaction temperature. An infrared lamp, a hot plate, theplasma jet itself or any other suitable heat source can be used as anadditional heat source, for example. The temperature monitoring can becarried out with common measuring processes, e.g. using infraredsensors, thermal sensors, etc.

A particularly advantageous measure provides that hydrogen is suppliedto the process chamber, in particular to the plasma jet. The hydrogen isin this respect supplied either into the plasma jet or into the processchamber. The supply of hydrogen has two advantages. On the one hand, thehydrogen serves as a catalyst for the diffusion process; on the otherhand, the hydrogen prevents the reoxidation of the substrate, forexample if the substrate is a zirconium oxide or another oxide. Thefunction as a catalyst results in that an additional energy supply takesplace in the boundary region of the substrate, in particular on theinterface, by the recombination of free hydrogen ions and the diffusionor the modification process is thus assisted.

As a further measure which optimizes the modification or the diffusion,the reaction temperature of the substrate is set to a value in the rangefrom 500-1200° C., in particular 800-1100° C. The setting andmaintaining of the reaction temperature in a predefined temperatureinterval is necessary because the quality and the process speed can thusbe fixed and influenced. It is moreover advantageous in order, forexample, to be able to ensure a uniform quality of the modification,that the substrate has a temperature distribution which is ashomogeneous as possible, which can be realized, for example, by the heatsource or by the plasma jet. Due to the increased temperatures, thediffusion depth of the reactive component into the boundary layer of thesubstrate is improved so that a particular advantage of the method inaccordance with the invention is a deeper penetration into the substrateand a greater thickness of the modified boundary layer resultingtherefrom.

It has proved to be particularly advantageous that the method isparticularly efficient when the plasma source and the substrate holderare moved relative to one another. It is furthermore advantageous if thesubstrate is held by a substrate holder, in particular by a tantalumwire. After the preheating of the substrate, in particular of theboundary surface, the modification of the boundary layer bounded by thesurface is started, with the plasma jet, which contains the reactivecomponent in dependence on the embodiment, preferably, but notnecessarily being led over the substrate by means of pivot movements.From case to case, in addition to or instead of pivoting the plasma jet,the substrate can be moved by means of the substrate holder or thesubstrate is moved by rotary or pivot movements relative to the plasmajet during the modification process. Al the named measures have theadvantage that a uniform treatment or coating is thereby achieved andpossible local hot spots and/or damage to the substrate surface, or tothe substrate, are avoided which may arise with a constantly alignedplasma jet with a high radiation power.

In another advantageous embodiment, the tantalum wire is used for fixingand holding the substrate so that the plasma and the reactive componentcompletely cover the substrate and a homogeneous modified boundary layeris created, with the tantalum wire not reacting with the reactivecomponent and being particularly suitable for use in an industrialproduction process.

It is furthermore advantageous if a controlled adjustment apparatus isprovided for the plasma source to control the direction of the plasmajet and/or the distance of the plasma source from the substrate, e.g. ina range from 0.05 m to 1 m, in particular in a range from 0.3 m to 0.6m. When the plasma source and thus the plasma jet are led over thesubstrate by means of pivot movements, that is moved with rotary orpivot movements relative to the substrate during the modificationprocess, a uniform treatment and modification of the boundary region isthereby achieved and possible local hot spots and/or damage to thesubstrate surface or to the substrate are avoided,

The distance of the plasma source from the substrate can amount, forexample, to 0.3 m-0.6 m, the pressure in the process chamber to 0.2 mbarto 1 mbar and the power which is supplied to the plasma source liesbetween 1 kW and 100 kW, in particular between 1 kW and 40 kW. Thepressure in the process chamber during the process amounts to between0.01 mbar and 100 mbar, in particular to between 0.01 mbar and 10 mar.In addition, the gas quantity flow of the reactive component during theprocess amounts to between 1 SLPM and 100 SLPM, in particular between 1SLPM and 10 SLPM for nitrogen or between 0.1 SLPM and 2 SLPM formethane. It becomes possible by a suitable choice of the parameters,e.g. the aforesaid parameters for preferred embodiments, to manufacturehigh-quality modified boundary regions in the treated substrates and toaccelerate the total process so advantageously that an efficient use inthe industrial sector is possible.

A main point of the method of the invention is thus the design of amodified LPPS process in the form of a diffusion method. It can thus bestated in summary that with the method presented of modifying a boundaryregion of the substrate, which is bounded by the surface of thesubstrate, the desired optical, physical and mechanical properties ofthe substrate can be efficiently manufactured as an industrialproduction method.

A specific embodiment will be presented again in the following whichagain illustrates a specific possible production method and which hasparticular importance for practice. In this respect, the individualmethod steps will again be briefly presented and explained withreference to an embodiment which provides the decorative modification ofthe boundary region of a ZrO₂ substrate with carbon C. The method inaccordance with the invention described in the following is in thisrespect in particular suitable for treating bracelets, for example forwristwatches, in order to give them, among other things, a desireddecorative appearance and color.

The following parameters are used in a specific embodiment of a methodin accordance with the invention, wherein the parameters or parameterranges used are indicated directly after the parameter, whereas furtherpossible parameters or parameter ranges are additionally indicated inparentheses.

-   Pressure in the process chamber 0.1 to 10 mbar (0.1 to 100 mbar)-   Gas quantity flow: 20 to 50 SLPM (2 to 200 SLPM)-   Phase gas mixture: Ar, He, H₂-   Reactive component: N₂ (1 to 10 SLPM) for nitriding-   Reactive component: CH₄ (0.1 to 2 SLPM) for carburizing, wherein any    molecular can be used which contains C-   The power supplied to the plasma source: 1 to 40 kW (1 to 100 kW)-   Spraying distance: 300 to 600 mm (50 to 1000 mm)-   Enthalpy: 2000 to 15,000 kJ/kg-   Plasma temperature: 2000 to 15,000 K-   Plasma speed y: 200 to 4000 m/s

If the method is used for the decorative modification of a substrate,e.g. of a bracelet or of a timepiece casing, the substrate, for examplea metal or a ceramic material, can possibly be advantageously polishedbefore the modification so that the substrate has e.g. a metallicappearance, for example gold colored, after the modification if theboundary region is nitrided or dark gray, for example, in the case ofcarburizing. Depending on the composition of the plasma jet, of thereactive component, of the position of the substrate, etc., there are nolimits to the color section here, for example blue, red, etc.Furthermore, other properties of the surface such as the scratchresistance, hardness, etc. can also be set directly by the use of amethod in accordance with the invention.

For fixing, the substrate is fastened to a substrate holder, preferablyby means of a tantalum wire, so that the plasma covers the wholesubstrate and the latter is uniformly modified. The tantalum wire inthis respect usually does not react with the reactive component so thatthe latter can be used over and over again. In addition, the substrateholder can be designed such that the latter, unlike the known classicalprocesses in which some few workpieces are arranged in a circle., can beused for several hundred workpieces. In addition, the plasma source andthe substrate are moved relative to one another to coat all workpiecesuniformly, with the reaction temperature preferably being regulated orcontrolled.

The heating of the substrate is an important part of the method sincethe quality and speed of the diffusion process are defined thereby. Itis important in this respect that the substrate has a temperaturedistribution which is as homogeneous as possible and which can liebetween 800 and 1100° C. to allow an ideal diffusion process. Theaddition of hydrogen to the plasma gas in this respect additionallyincreases the energy density in the boundary region and on the surfaceof the substrate. Due to the elevated temperatures, the hydrogen issplit into individual hydrogen ions which recombine to H₂ again, whereinthe process of recombination usually takes place in the boundary regionand at the surface of the substrate, whereby the modification processcan be further improved.

After the preheating of the substrate, the actual modification processis started. For this purpose, the reactive component, for examplemethane CH₄ or nitrogen N₂ or another gas, liquid, suspension or solidis, for example, injected into the plasma source. The reactive componentis split due to the energy of the plasma jet so that the required atoms,for example carbon C, nitrogen N or others are formed and can diffuseinto the boundary region of the substrate. The modification, that is thediffusion per se, takes a few minutes (approximately 30 minutes), with aprocess also being possible including a plurality of steps (e.g. approx.2×15 minutes), which further improves the diffusion process. The processin accordance with the invention is thus approximately 6 times fasterthan all known classical diffusion processes such as the classicalnitriding, which often require 3 hours or more.

In a particularly preferred embodiment, the reactive component carbon C,which arises by injection of methane into the plasma source, is diffusedinto the crystal structure of the boundary region of the zirconiumdioxide ZrO₂ and replaces the oxygen O₂ at least in part, with zirconiumcarbide ZrC being formed. For this purpose, the CH₃ molecule is firstsplit by means of the plasma jet and the boundary region of thesubstrate is heated to reaction temperature and the required reactionenergy is provided so that the carbon C can diffuse into the boundaryregion. It is understood that the reactive gas can also be activated orsplit e.g. by contact with the hot substrate surface or similar. Settingthe correct reaction temperature is in particular important since thediffusion process only runs ideally as a rule in a specific temperaturerange since, if the substrate is too hot, the diffusion process does notrun ideally, and can e.g. at least partly be suppressed or reversed or,if the substrate is too cold, the reactive component, that is carbon,nitrogen or another reactive component, cannot diffuse deeply enoughinto the boundary region.

As already described, the CH₄ is first injected into the plasma source,for example into a nozzle of the plasma source. The methane cantherefore also be injected or otherwise introduced into the plasma jetor into the plasma chamber, with the reactive component in this casehaving a comparatively large free mean path distance and coming intocontact with the plasma jet very quickly. As soon as the molecules ofthe reactive component come into contact with the high-energy plasmajet, the molecule can be split and as a rule different split products,that is CH, C, H₂, can be found in the plasma jet.

The substrate is heated before the actual modification process to thecorresponding reaction temperature which lies for zirconium dioxide e.g.preferably between 800 and 1100° C. and this range of the reactiontemperature is maintained using the plasma jet during the modification,that is during the diffusion process.

A similar process results when nitrogen N is used instead of carbon C;in this case zirconium nitride is formed which has a yellow color. Themethod can also be used on other substrates, for example titanium, andalso many other materials.

After the modification procedure, the substrate can be cooled in asluice and simultaneously the processing of the next workpiece can bestarted to further increase the productivity and efficiency of themethod and to maintain the evacuated state of the process chamber. Thecooling process had a duration of 15 minutes in the present embodiment.

FIGURES

The invention will be explained in more detail in the following withreference to the drawing. There are shown in a schematic representation:

FIG. 1 a plasma modification plant in which the method in accordance hthe invention can be carried out; and

FIG. 2 a modified substrate.

DETAILED DESCRIPTION

FIG. 1 shows an embodiment of a plasma modification plant 1 for themodification of a boundary region 9 of a substrate 3 bounded by asurface 10, wherein an evacuated process chamber 2 is provided having aplasma source 4 for generating a directed plasma jet 5, whereinfurthermore a reactive component is supplied into the process chamber 2with a flow of predefined size and the substrate 3 is heated to apredefined reaction temperature. The reactive component isdiffusion-activated in accordance with the invention by the directedplasma jet 5 so that the reactive component diffuses into the boundaryregion 11 of the substrate 3 at a predefined diffusion rate.

The plasma coating plant 1 includes a process chamber 2 having a plasmasource 4 for generating a plasma jet 5, a controlled pump apparatus,which is not shown in FIG. 1 and which is connected to the processchamber 2 to set the pressure in the process chamber 2, and a substrateholder 8 for holding the substrate 3.

The pressure in the process chamber 2 is set to a predefined value bymeans of the controlled pump apparatus, with the plasma modificationplant 1 additionally including an injection apparatus (not shown) tointroduce, in particular to inject, at least one reactive component intothe plasma jet 5 or into the process chamber 2 in liquid and/or gaseousand/or powder form and/or as a suspension.

If required, the substrate holder 8 can be designed as a displaceablebar holder to move the substrate out of a pre-chamber through a sluice 9into the process chamber 2. The bar holder additionally makes itpossible to rotate the substrate 3, if necessary, during themodification. In practice, normally a plasma source 4 is therefore usedwhich is usually used for thermal plasma spraying. Typically, the plasmasource 4 is connected to a power supply, for example to a DC supply fora DC plasma torch, and/or to a cooling apparatus and/or to a plasma gassupply and is connected, from case to case, to a supply having thereactive component and/or a conveying apparatus for powdery reactivecomponents or suspensions.

A conventional plasma source 4 for thermal spraying can include, forexample, an anode and a cathode to generate an electrical discharge,wherein the anode and the cathode are normally cooled, for example bymeans of cooling water, in the power range required for thermalspraying.

A process gas, also called a plasma gas, supplied to the plasma source 4is ionized in the electrical discharge to generate a plasma jet 5 havinga temperature of up to 20,000 K. The plasma jet 5 exits the plasmasource 4 at a speed of typically 200 m/s to 4000 m/s. The process gas orplasma gas can, for example, include argon, nitrogen, helium and/orhydrogen or a mixture of a noble gas with nitrogen and/or hydrogen orcan be composed of one or more of these gases.

FIG. 2 schematically shows a modified substrate 3 after the use of themethod in accordance with the invention.

As can be seen, a modified boundary region 11 is located in a boundaryregion 9 of the substrate 3, which is a bracelet or a housing for atimepiece, bounded by the surface 10 of the substrate 3. Themodification of the boundary region 9 is in this respect formed by meansof the method in accordance with the invention, that is by means ofdiffusion of the reactive component into the boundary region 11 andsubsequent interaction of the reactive component with the substrate 3.

1. A method of modifying a boundary region of a substrate bounded by asurface, wherein an evacuated process chamber is provided having aplasma source for generating a directed plasma jet, and whereinfurthermore a reactive component is supplied into the process chamberwith a flow of a predefined size, and wherein the substrate is heated toa predefined reaction temperature, and wherein the reactive component isdiffusion-activated by the directed plasma jet such that the reactivecomponent diffuses into the boundary region of the substrate at apredefinable diffusion rate.
 2. A method in accordance with claim 1,wherein the reactive component is liquid and/or gaseous and/or powderyand/or a suspension.
 3. A method in accordance with claim 1, wherein thereactive component is injected into the plasma jet for the diffusionactivation in the plasma source and/or is injected into the free plasmajet and/or is injected into the process chamber.
 4. A method inaccordance with claim 1, wherein the reactive component, which includesa hydrocarbon compound and/or oxygen and/or nitrogen, reacts with thesubstrate in the boundary region of the substrate, and in so doing acompound is created in the boundary region.
 5. A method in accordancewith claim 1, wherein the process chamber includes a heat source to beable to carry out the modification at a reaction temperature within apredefined temperature range.
 6. A method in accordance with claim 1,wherein the substrate is preheated to the reaction temperature by meansof the additional heat source and/or the reaction temperature iscontrolled or regulated by means of the plasma jet during themodification.
 7. A method in accordance with claim 1, wherein hydrogenis supplied to the process chamber.
 8. A method in accordance with claim1, wherein the reaction temperature of the substrate is set to a valuein the range of 800-1200° C.
 9. A method in accordance with claim 1,wherein the plasma source and a substrate holder are moved relative toone another.
 10. A method in accordance with claim 1, wherein thesubstrate is held by the substrate holder.
 11. A method in accordancewith claim 1, wherein a controlled adjustment apparatus is provided forthe plasma source to control the direction of the plasma jet and/or thedistance of the plasma source 4)-from the substrate, in a range from0.05 m to 1 m.
 12. A method in accordance with claim 1, wherein thepower which is supplied to the plasma source lies between 1 kW and 100kW.
 13. A method in accordance with claim 1, wherein the pressure in theprocess chamber during the method amounts to between 0.01 mbar and 100mbar,
 14. A method in accordance with claim 1, wherein the gas quantityflow of the reactive component during the process amounts to between 1SLPM and 100 SLPM.
 15. The method in accordance with claim 14, whereinthe gas quantity flow of the reactive component during the processamounts to between 1 SLPM and 10 SLPM for nitrogen or between 0.1 SLPMand 2 SLPM for methane.